A reliable objective test is still needed to confirm the diagnosis of neurogenic thoracic outlet syndrome (NTOS). Over the past 20 years, it has been suggested that responses to medial antebrachial cutaneous nerve (MAC) and C8 nerve root stimulation could be used for this purpose. Herein, we explore this thesis. A clinical diagnosis of NTOS was established in 41 patients, all of whom underwent surgical decompression. Preoperatively, all patients were studied with MAC sensory neural action potential (SNAP) determinations and C8 nerve root stimulation. Controls were 19 asymptomatic, healthy volunteers. MAC sensory latency on 79 control sides was 1.5-2.4 msec, while latency in 41 symptomatic patients ranged 2.2-2.8 msec. Latency of 2.5 or greater was noted in 30 patients (specificity 99%, sensitivity 73%), confirming a diagnosis of NTOS, while the remaining 11 (27%) fell into the borderline zone of 2.2-2.4 msec. The latency difference between right and left sides in controls was 0-0.2 msec in 17 (89%), while in NTOS patients 31 had a difference of 0.3 msec or more (sensitivity 89%, specificity 63%). Amplitudes of 10 muV or more occurred in 77 of 79 control sides, whereas the amplitude was under 10 muV in 28 patients (specificity 97%, sensitivity 68%). Amplitude ratios between right and left sides in controls were 1.7 or less. Ratios of 2.0 or more were measured in 25 patients (specificity 100%, sensitivity 61%). Using the four diagnostic criteria (latency over 2.4 msec, latency difference between sides of 0.3 or more, amplitude under 10 muV, and amplitude ratios of 2.0 or more), 40 of the 41 patients had at least one of the four diagnostic criteria, 23 patients (56%) had three or four positive criteria, and 12 (29%) had two. C8 nerve root stimulation responses were below normal (56 M/sec) in 54%. MAC measurement is a fairly reliable technique for confirming the diagnosis of NTOS. Latency determination appeared to be a slightly more consistent measurement in this study, but amplitude and C8 nerve root stimulation were also helpful. A combination of these techniques seems to be the most reliable approach.